Demand for digital cameras has dramatically increased in the consumer market place for various reasons. One reason for this demand is the fact that digital camera produce digital photos, which are readily distributable through an electronic medium, such as the Internet. Thus, friends and families can exchange digital photos as easily as sending and receiving emails. Another reason for the increased demand is that digital photos produced by the digital cameras can be enhanced and manipulated using an image processing software. Still another reason for the demand is that digital cameras include features that make the digital cameras more attractive than conventional cameras, such as a preview of captured images and an option to delete unwanted images.
A digital camera typically employs a solid-state image sensor, either a Charge Coupled Device (CCD) sensor or a Complementary Metal Oxide Semiconductor (CMOS) sensor, to digitally capture a scene of interest. A solid-state image sensor includes an array of photosensitive pixels. Each photosensitive pixel corresponds to an image pixel of a digitally captured image. Thus, the resolution of digitally captured image is dependent on the number of photosensitive pixels included in the image sensor. Consequently, there is a growing interest to develop image sensors with increased number of photosensitive pixels.
A problem with solid-state image sensors is that the sensors may include a significant number of defective photosensitive pixels. Defective photosensitive pixels may be classified as “hot pixels” or “cold pixels”. Hot pixels produce unnaturally bright image pixels in digitally captured images, which are caused by leakage current in the photosensitive pixels during an integration period, i.e., the exposure period to capture a scene of interest. Thus, hot pixels may result in a digital image with bright artifacts due to isolated bright image pixels on a dark background. In contrast to the hot pixels, cold pixels produce unnaturally dark image pixels in digitally captured images. Cold pixels are non-responsive photosensitive pixels that fail to accumulate sufficient charge during an integration period. Thus, cold pixels may result in a digital image with dark artifacts due to isolated dark image pixels on a bright background. The number of these artifacts is likely to increase as the density of photosensitive pixels is increased in solid-state image sensors, since more defective photosensitive pixels are expected to be found in image sensors with higher density of photosensitive pixels. Thus, the bright and dark artifacts caused by the hot and cold pixels pose a greater problem of image degradation for mega-pixel sensors.
One conventional approach to alleviate the problem of hot and cold pixels is to map the defective photosensitive pixels in each image sensor by testing all the photosensitive pixels after manufacturing the image sensor. The map of the defective photosensitive pixels can then be used to correct the erroneous image signals produced by the defective photosensitive pixels. Thus, each digital camera is customized to correct the image signals from the mapped defective photosensitive pixels of a specific image sensor. However, this approach introduces complex testing and customization processes, which increase the overall manufacturing cost of the digital cameras.
Another conventional approach to alleviate the problem of hot and cold pixels is to detect and correct the erroneous image signals from defective photosensitive pixels using an algorithm after the analog image signals have been converted to digital image signals. An advantage of this approach is that the same algorithm can be used in all the digital cameras. Thus, customization of the digital cameras in accordance with each image sensor is not required. However, this approach does require a significant amount of processing resources for the algorithm, which tends to increase the power usage of the digital cameras and tends to slow down the operation of the digital cameras to process digitally captured images.
In view of the disadvantages of the conventional approaches to alleviate the problem of hot and cold pixels, there is a need for a system and method for efficiently correcting image signals from defective photosensitive pixels.